The present invention relates to cataphoretically coating the surfaces of electrically conducting substrates acting as cathodes in an electrical dip lacquering process and a coating bath therefor based on aqueous solutions and/or aqueous dispersions of salts of cationic film forming agents with organic and/or inorganic acids. The present invention furthermore relates to using the coating bath for making cataphoretic coatings and to using anodes of stainless steel.
The state of the art of cataphoretic coating of metallic surfaces and the bath compositions used therein may be ascertained by referring to U.S. Pat. Nos. 3,230,162; 3,455,806; 3,682,814; 3,922,212; 3,984,299 and 4,001,155 the disclosures of which are incorporated herein.
As known to the prior art, cathode coatings are deposited in an electrical dip bath by passing a DC current between an anode and a cathode immersed in the bath. The electrical dip or coating bath contains aqueous solutions or aqueous dispersions of salts of cationic film forming agents with organic and/or inorganic acids, which may furthermore contain the conventional pigments and fillers. The pH value of the dip bath as a rule is set between 1 and 9. Under the influence of the DC current, the cationic film forming agent deposits on the substrate acting as cathode, while the acids used for neutralization migrate to the anode and concentrate there.
To date, only graphite electrodes have proved to be free from difficulties as anodes. The use of iron or steel electrodes as anodes as a rule cannot be practically implemented because these metals decay and dissociate relatively quickly. The dissociating iron furthermore interferes with the coating bath, whereby the deposition of the cationic film forming agent on the cathode is adversely affected.
Again so-called stainless steel electrodes with higher resistance to corrosion are not applicable in every case because they are corroded by pitting, especially in those cationic coating baths containing chloride ions. This pitting in the end results in complete dissociation of the stainless steel anodes too. It has been found that about 0.25 mg. of iron per coulomb are dissolved in an electrical dip basin containing chloride ions. The theoretical value for complete dissociation is about 0.29 mg. of iron lost per coulomb. In practice, this means that stainless steels too will dissolve just as quickly in constant use as simple iron or steel electrodes unless their application is in baths extensively free of chloride ions. The especially degrading chloride ions reach the coating baths through the use of hydrochloric acid as the neutralizing means, or also as impurities from the industrially prepared synthetic resins, pigments and fillers. Thus the epoxy resins used in practice contain chloride ions. Such a concentration of chloride ions obviously do not interfere when the coating means are applied in a form other than electrical dip lacquering means.